The conductivity, mobility, photoconductivity and photo response measurements in GaS0.75Se0.25 mixed crystals were carried out in the temperature range of 150-450 K. The room temperature conductivity, mobility and electron concentration values were 10(-9) (Omega-cm)(-1), 48 cm(2)V(-1) s(-1) and similar to 10(9) cm(-3), respectively. Two donor levels were obtained from temperature-dependent conductivity and carrier concentration, located at energies of about 755 and 465 meV below the conduction band. Single donor-single acceptor analysis yields the same donor level at 465 meV with donor and acceptor concentrations of 8.7x10(14) and 5.3x10(13) cm(-3), respectively. The mobility-temperature dependence shows that ionized impurity scattering dominates the conduction up to the temperature 3 10 K with different temperature exponent, while above this critical temperature; the phonon scattering is dominant conduction mechanism. From the photo-response spectra, the maximum photocurrent was observed for all the samples at 2.42 eV, and varied slightly with temperature. Moreover, the photocurrent-light intensity dependence in these crystals obeys the power law, I-ph proportional to phi(gamma) with gamma between 1.7 and 2.0 for various applied fields and temperatures. (c) 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.